The invention concerns an alarm system in accordance with the general concept set forth herein below, in which a number of alarm devices, which are located on two-lead signal lines and which can have different electrical states, are connected with a signal center, where the signals transmitted by the individual alarm devices can be evaluated to obtain differentiated malfunction or fault warning alarm signals.
Automatic alarm systems have the purpose of recognizing danger, e.g. fires or unauthorized entries, as early as possible, in order to allow an effective defense. The alarm devices used in automatic alarm systems generally have at least one sensor which translates danger parameters, e.g. an increased temperature, the appearance of gas or smoke, a break-in, etc., into an electrical value. The alarm devices further contain at least one switching element which forms a threshold value to establish an alarm threshold. If the sensor output signal exceeds this threshold, an electrical converter present in the alarm device is triggered and the electrical status of the alarm device (voltage, current, impedance) changes suddenly. This change in status of the alarm device is transmitted to the signal center and evaluated and displayed there. An alarm device therefore generally has two states, the inactive state and the alarm state. However, there are also alarm systems known where the alarm devices are equipped with automatic monitoring devices which indicate a defect. In these systems, the alarm devices can therefore also have a third state, the so-called trouble or malfunction state. The main advantage of such alarm systems consists in the simple and reliable transmission of the signals.
Contradictory demands are placed on such alarm systems. On the one hand, they are intended to report danger at the earliest possible stage, in order to be able to initiate appropriate defense measures. For this purpose, highly sensitive sensors are used to recognize danger parameters, e.g. ionization smoke detectors or passive infrared alarms, etc. On the other hand, alarm systems are supposed to function with the greatest possible reliability, i.e. they are supposed to go off only when genuine danger is present. If such highly sensitive alarm devices are utilized, it nevertheless frequently occurs that disturbances cause an alarm signal, even though there is no danger present. Such malfunction can result in costly defense measures being activated unnecessarily, e.g. the police or the fire department may be called.
To overcome this disadvantage, therefore, alarm systems have been proposed, e.g. in Swiss CH-PS No. 547,532, where a pre-alarm signal is issued by a second threshold detector with a lower threshold value. During the time span between the pre-alarm signal and the alarm signal, it is possible to check whether there is real danger or whether there is a malfunction. In addition, alarm devices which tend to give off false alarms can thereby be recognized and replaced at an early stage, since the pre-alarm signal can also be interpreted as a malfunction or fault signal.
There have also already been proposals made for alarm systems in which various time delay switches would be activated after the alarm threshold is exceeded, in order to be able to check, before an alarm is sounded, whether there is a real danger situation. It is clear, however, that these alarm systems cannot meet the demand for an early alarm in case of danger.
Another disadvantage of the known alarm devices is that most are necessarily exposed to ambient contamination. This creates a risk that the sensor output signal will slowly be altered. This can have the result that the alarm devices either become blocked or have an increasing probability of giving off a false alarm. Therefore, alarm devices have been proposed in which the alarm threshold is slowly changed in accordance with the drift of the sensor output signal. Thus the distance between the inactive value and the alarm value remains constant within certain limits, which can extend the useful life of such alarms, particularly under severe ambient conditions. However, there remains the difficulty of recognizing the magnitude of the drift in the signal center.
In order to overcome the disadvantages mentioned, it has been proposed that instead of transmitting an alarm signal, the measurement value analog of the danger parameter to be measured should be transmitted to the signal center. Hence, the decision as to whether this is a real danger or a malfunction would be made in the signal center when a more accurate evaluation can be made on the basis of a comparison of the measurement values of different alarm devices.
In the method for transmission of measurement values in a fire alarm system proposed in W. German DE-PS No. 2,533,382, for example, measurement values determined by individual fire alarm devices attached to the alarm lines in a chain are transmitted as an analog signal to a signal center, and are there connected to obtain more differentiated malfunction or alarm reports, with all the fire alarms being cut off from the alarm line by a voltage change at the beginning of repetitive inquiry cycles, and then being turned on again in a pre-determined sequence. Each individual fire alarm also connects the subsequent fire alarm device to the line voltage, after a time delay corresponding to its own measurement value, and in the signal center, the alarm address in each case is determined from the number of prior increases in the line current, and the levels of the individual measurement values are deduced from the length of the switching delay in each case.
It is a known fact, however, that the demands for transmission reliability become greater and greater the more information there is to be transmitted. The increasing electromagnetic contamination of our environment particularly has the result that the transmission of measurement values from the alarm device to the signal center is frequently disturbed. Although it is possible to recognize transmission errors using relatively simple means, this does require a certain amount of time, and precisely in the case of alarm systems this time is not available.
Therefore, there is a need for alarm systems which make it possible to transmit more information from the alarm device to the signal center, without having to accept the disadvantages of complicated transmission systems. The alarm systems in which a maximum of three states are reported therefore stand out on account of the great stability and reliability of the transmission caused by the transmission simplicity.